For all their innovations, NuScale Power’s small modular reactors remain conventional in one way: They would use ordinary uranium-based reactor fuel that’s meant to be used once and safely disposed of. But for decades, nuclear engineers envisioned a world powered by “fast reactors” that can breed an essentially boundless supply of plutonium that can be reprocessed into fuel. Early in the atomic age, experts believed nuclear energy would one day supply most of the world’s power, raising the specter of a uranium shortage and boosting interest in fast breeder reactors.

However, the reactors are complex and must be cooled with substances such as liquid sodium or molten salt. The chemically intensive recycling process produces plenty of its own hazardous waste. And the closed fuel cycle also would establish a global market for plutonium, the stuff of atomic weapons, raising proliferation concerns. Perhaps most important, the world is in no danger of running out of uranium. So some experts doubt fast reactors will ever become mainstream.